U.S. patent number 5,128,686 [Application Number 07/572,306] was granted by the patent office on 1992-07-07 for reactance buffered loop antenna and method for making the same.
This patent grant is currently assigned to Motorola, Inc.. Invention is credited to Robert Kurcbart, William Tan.
United States Patent |
5,128,686 |
Tan , et al. |
July 7, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Reactance buffered loop antenna and method for making the same
Abstract
A reactance buffer maintains a substantially constant reasonant
frequency for an adjustable size loop antenna having first and
second antenna segments. Each segment has first and second ends,
the first ends being coupled to a receiver, and the second ends
providing loop size adjustment. The reactance buffer comprises a
reactance buffer input coupled to the second end of the first
antenna segment. A plurality of taps are linearly disposed along a
longitudinal axis of the first wristband section, and has a
predetermined length between the outermost taps corresponding to
the loop antenna size adjustment required. The taps provide
selectable reactance buffer outputs for coupling to the second end
of the second antenna segment. A plurality of reactance elements
are arranged non-serially and couple the reactance buffer input to
each of the plurality of taps and provide a substantially constant
reactance measured between the reactance buffer input and each of
the plurality of taps.
Inventors: |
Tan; William (Lantana, FL),
Kurcbart; Robert (Boca Raton, FL) |
Assignee: |
Motorola, Inc. (Schaumburg,
IL)
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Family
ID: |
26971130 |
Appl.
No.: |
07/572,306 |
Filed: |
August 27, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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299276 |
Jan 23, 1989 |
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Current U.S.
Class: |
343/718; 333/24R;
343/744; 343/868 |
Current CPC
Class: |
H01Q
1/273 (20130101); H01Q 7/02 (20130101) |
Current International
Class: |
H01Q
1/27 (20060101); H01Q 7/00 (20060101); H01Q
7/02 (20060101); H01Q 007/00 () |
Field of
Search: |
;343/718,741,743,744,868,866,870,871,702 ;333/17.3,32,24R,24C
;455/274,290,292,344,349,351,193 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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196602 |
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Dec 1982 |
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JP |
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212015 |
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Oct 1985 |
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JP |
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Primary Examiner: Wimer; Michael C.
Attorney, Agent or Firm: Macnak; Philip P. Ingrassia;
Vincent B. Koch; William E.
Parent Case Text
This is a continuation of application Ser. No. 07/299,276, filed
Jan. 23, 1989, now abandoned.
Claims
We claim:
1. A reactance buffer, for maintaining a substantially constant
resonant frequency for a loop antenna having a plurality of
selectable loop circumferences, the loop antenna formed from first
and second conductor antenna segments, each antenna segment having
first and second ends, the first ends of each antenna segment being
coupled to a receiver input, the second ends of each antenna
segment being selectably coupled for providing adjustment of the
loop circumference, said buffer comprising:
a reactance buffer input coupled to the second end of the first
antenna segment;
a plurality of taps, linearly disposed along a longitudinal axis of
the reactance buffer, the distance between the outermost of said
plurality of taps providing a predetermined length corresponding to
the length of adjustment of the loop circumference, said taps
providing selectable coupling positions for coupling to the second
end of the second antenna segment; and
a plurality of reactance elements, arranged non-serially between
said reactance buffer input and each of said plurality of taps,
wherein one or more of said reactance elements are coupled between
said input and a corresponding one of said taps to provide a
substantially constant reactance when measured between said
reactance buffer input and each of said plurality of taps,
whereby the resonant frequency of the loop antenna remains
substantially constant when the loop circumference is adjusted.
2. The reactance buffer according to claim 1, wherein said
reactance elements are inductive elements.
3. The reactance buffer according to claim 2, wherein the magnitude
of the inductance measured between the buffer input and each tap is
substantially constant.
4. The reactance buffer according to claim 2, wherein each of said
inductive elements is formed from a conductor.
5. The reactance buffer according to claim 4, wherein said
conductors are formed from sheet metal.
6. The reactance buffer according to claim 5, wherein said sheet
metal is selected from a group of sheet metals consisting of
copper, beryllium copper, and nickel silver.
7. The reactance buffer according to claim 1, wherein said
reactance elements comprise a plurality of paired inductive and
capacitive elements defining inductor/capacitor pairs.
8. The reactance buffer according to claim 7, wherein said
inductive elements are coupled in series with said buffer
input.
9. The reactance buffer according to claim 7, wherein each of said
inductive elements if formed from a conductor.
10. The reactance buffer according to claim 9, wherein said
conductors are formed from sheet metal.
11. The reactance buffer according to claim 10, wherein said sheet
metal is selected from a group of sheet metals consisting of
copper, beryllium copper, and nickel silver.
12. The reactance buffer according to claim 7, wherein said
capacitive elements are fixed value capacitors.
13. The reactance buffer according to claim 12, wherein each
inductor of each inductor/capacitor pair has an input and an output
terminal, each of said capacitors of each inductor/capacitor pair
has an input and an output terminal, each capacitor input terminal
being coupled to a respective inductor output terminal, and each
capacitor output terminal being coupled to a respective one of said
plurality of taps.
14. The reactance buffer according to claim 13, wherein the
magnitude of 2.pi.fL.sub.cum +1/(2.pi.fC.sub.tap) measured from
said buffer input to each of said plurality of taps is
substantially constant, f being the frequency of operation,
L.sub.cum being the cumulative inductance associated with each of
said plurality of taps, and C.sub.tap being the tap capacitance of
each of said inductor/capacitor pairs coupled to each of said
plurality of taps.
15. The reactance buffer according to claim 4, wherein the
inductance of each inductive element is controlled by the conductor
geometry.
16. The reactance buffer according to claim 9, wherein the
inductance of each inductive element is controlled by the conductor
geometry.
17. A buffered loop antenna, having a plurality of selectable loop
circumferences, the loop antenna being coupled to a receiver having
signal and ground inputs coupled to an antenna resonating capacitor
for resonating the loop antenna to a predetermined frequency, said
loop antenna comprising:
a first conductor, having a first end coupled to the receiver
signal input and a second end, said first conductor forming a first
portion of the loop antenna;
a second conductor, having a first end coupled to the receiver
ground and a second end, said second conductor forming a second
portion of the loop antenna;
reactance buffer means, comprising
a reactance buffer input coupled to said second end of said first
conductor,
a plurality of taps linearly disposed along a longitudinal axis of
said reactance buffer the distance between the outermost of said
plurality of taps providing a predetermined length corresponding to
the loop circumference adjustment, and
a plurality of reactance elements, arranged non-serially between
said reactance buffer input and each of said plurality of taps,
wherein one or more of said reactance elements are coupled between
said input and a corresponding one of said taps to provide a
substantially constant reactance when measured between said
reactance buffer input and each of said plurality of taps; and
coupling means, coupled to said second end of said second
conductor, for coupling said second conductor to any of said
plurality of taps,
whereby the resonant frequency of the loop antenna remains
substantially constant when any of said plurality of taps is
selected to adjust the loop circumference.
18. The buffered loop antenna according to claim 17 wherein said
first and second conductors are sheet metal.
19. The buffered loop antenna according to claim 18 wherein said
sheet metal is selected from a group consisting of copper,
beryllium copper, and nickel silver.
20. A wristband loop antenna for a wrist worn electronic device,
the device including a receiver having signal and ground inputs
coupled to an antenna resonating capacitor for resonating the loop
antenna to a predetermined frequency, said wristband loop antenna
comprising:
a first wristband section, including
a first conductor, having a first end for coupling to the receiver
signal input and a second end, said first conductor forming a first
portion of the loop antenna within said first wristband second,
and
reactance buffer means, comprising
a reactance buffer input coupled to said second end of said first
conductor,
a plurality of taps linearly disposed along a longitudinal axis of
said reactance buffer the distance between the outermost of said
plurality of taps providing a predetermined length corresponding to
the loop antenna diameter adjustment, and
a plurality of reactance elements, arranged non-serially between
said reactance buffer input and each of said plurality of taps,
wherein one or more of said reactance elements are coupled between
said input and a corresponding one of said taps to provide a
substantially constant reactance when measured between said
reactance buffer input and each of said plurality of taps; and
a second wristband section including
a second conductor, having a first end coupled to the receiver
ground and a second end, said second conductor forming a second
portion of the loop antenna within said second wristband section,
and
coupling means, coupled to said second end of said second
conductor, for coupling said second conductor to any of said
plurality of taps, whereby when the wristband length is adjusted by
selecting any of said plurality of taps, the resonant frequency of
said loop antenna remains substantially unchanged.
21. The wristband antenna according to claim 20 wherein said first
wristband section further includes compliant top and bottom members
for enclosing said first conductor and said reactance buffer means,
and said second wristband section further includes compliant top
and bottom members for enclosing said second conductor.
22. The wristband antenna according to claim 21 wherein said top
and bottom members are molded from a urethane rubber.
23. The wristband antenna according to claim 21 wherein said top
and bottom members are die cut from leather.
24. The wristband antenna according to claim 20 wherein said first
and second wristband sections further include attachment means for
attaching said wristband sections to the electronic device.
25. The wristband antenna according to claim 20 wherein said first
and second conductors are sheet metal.
26. The wristband antenna according to claim 25 wherein said sheet
metal is selected from a group consisting of copper, beryllium
copper, and nickel silver.
27. A wrist worn receiving device, comprising:
a receiver located within a housing; and
a wrist band, including first and second wrist band sections
coupled to said housing, for securing the housing to a users
wrist,
said first wristband section forming a first portion of a loop
antenna, and including a reactance buffer having an input coupled
to said receiver, and a plurality of outputs linearly disposed
along a longitudinal axis of said first wristband section opposite
said housing, said reactance buffer comprising a plurality of
reactance elements, arranged non-serially between said reactance
buffer input and each of said plurality of outputs, wherein one or
more of said reactance elements are coupled between said input and
a corresponding one of said outputs for providing a substantially
constant reactance measured between said reactance buffer input and
each of said reactance buffer outputs,
said second wristband section forming a second portion of the loop
antenna, and including a coupling means, coupled to said receiver
and affixed to said second wristband section opposite said housing,
said coupling means providing selective coupling to said plurality
of reactance buffer outputs when securing the housing to the users
wrist,
whereby the length of said wristband is freely adjustable to fit
the users wrist by coupling said coupling means to a corresponding
one of said plurality of reactance buffer outputs, and whereby the
resonant frequency of said loop antenna remains substantially
unchanged when said coupling means is coupled to any of said
plurality of outputs.
28. The wrist worn receiving device according to claim 27, wherein
each of said reactance elements are formed from a conductor.
29. The wrist worn receiving device according to claim 28, wherein
said conductors are formed from sheet metal.
30. The wrist worn receiving device according to claim 29, wherein
said sheet metal is selected from a group of sheet metals
consisting of copper, beryllium copper, and nickel silver.
31. The wrist worn receiving device according to claim 27 wherein
said first and second wrist band sections further include
attachment means for coupling said first and second wrist band
sections to said housing.
32. The wrist worn receiving device according to claim 27 wherein
said first wrist band section further includes a first conductor
coupled between said reactance buffer input and said receiver to
form the first portion of the loop antenna, and further wherein
said second wrist band section further includes a second conductor
coupled between said coupling means and said receiver to form the
second portion of the loop antenna.
33. The wrist worn receiving device according to claim 32 wherein
said first and second conductors are sheet metal.
34. The wrist worn receiving device according to claim 33 wherein
said sheet metal is selected from a group consisting of copper,
beryllium copper, and nickel silver.
35. The wrist worn receiving device according to claim 27 wherein
said first and second wrist band sections are molded from a
urethane rubber.
36. The wrist worn receiving device according to claim 27 wherein
said first and second wrist band sections are die cut from
leather.
37. The wrist worn receiving device according to claim 27 wherein
said coupling means is a conductive clasp for providing adjustment
of the wrist band length and electrical coupling of the first and
second antenna portions.
Description
FIELD OF THE INVENTION
This invention relates generally to the field of loop antennas, and
more particularly to a reactance buffered loop antenna suitable for
use as a wristband antenna for a wrist worn electronic device.
DESCRIPTION OF THE PRIOR ART
As electronic circuits have been miniaturized, and in particular
receivers, it has become possible to package the electronics into
housings suitable to be comfortably worn on the wrist. Antennas
used with these wristworn receivers have often utilized simple
single turn loop antennas which have been incorporated into the
wristband of the device. Such an antenna generally used a
nonstretchable two-piece wristband is shown in FIG. 1. Rivets, or
similar fasteners, were used to provide a series of regularly
spaced holes in one of the wristband sections required to
accommodate the varying sizes of the human wrist, often providing
the electrical connection to close the loop when the wristband was
fastened to the wrist. Since the inductance of such a loop antenna
is dependent upon the physical geometry of the loop antenna, such
as loop diameter or length, the tuning of such a loop antenna
varied with the wrist size. Consequently, when the loop antenna was
tuned for a particular wrist size, increasing or decreasing the
loop diameter by increasing or decreasing the loop length, as would
happen when an adjacent contact point was selected when strapping
the device to the wrist, resulted in substantial changes in the
antenna's resonant frequency and correspondingly substantial
changes in the receiver's sensitivity. As a consequence, factory
pretuning of such a wristband loop antenna was not possible.
Commercialization of such wrist worn receivers was consequently
limited to retailers employing skilled technicians capable of
tuning the antennas on the devices as they were sold. As noted,
even this did not guarantee antenna performance when the wearer was
inconsistent in strapping the device to the wrist.
Other antenna structures have also been proposed for use in
wristworn receivers. One such wristband antenna consisted of a
number of ferrite antenna links affixed to a rigid wristband.
Another wristband antenna consisted of conductors incorporated into
a wristband so as to allow a stretchable wristband. Both types of
antennas exhibited the same tuning problems as the non-stretchable
wristband antenna. As the geometry of the loop was changed, and
depending upon the position on the wrist, detuning and reduced
receiver sensitivity would occur.
SUMMARY OF THE INVENTION
A reactance buffer is described for maintaining a substantially
constant resonant frequency for an adjustable size loop antenna
having a plurality of selectable loop circumferences, the loop
antenna formed from first and second conductor antenna segments,
each antenna segment having first and second ends, the first ends
of each antenna segment being coupled to a receiver input, the
second ends of each antenna segment being selectably coupled
providing adjustment of the loop circumference. The reactance
buffer comprises a buffer input coupling to the second end of the
first antenna segment. A plurality of taps are linearly disposed
along a longitudinal axis of the reactance buffer, the distance
between the outermost of said plurality of taps providing a
predetermined length corresponding to the length of adjustment of
the loop circumference. The taps provide selectable coupling
positions for coupling to the second end of the second antenna
segment. A plurality of reactance elements arranged non-serially
between the reactance buffer input and each of the plurality of
taps when one or more of the reactance elements are coupled between
the input and a corresponding one of the taps to provide a
substantially constant reactance when measured between the
reactance buffer input and each of the plurality of taps.
A wristband loop antenna is described for a wristworn electronic
device which includes a receiver having signal and ground inputs
coupled to an antenna resonating capacitor for resonating the loop
antenna to a predetermined frequency. The wristband loop antenna
comprises first and second wristband sections. The first wristband
section includes a first conductor having a first end coupled to
the receiver signal input and a second end. The first conductor
forms a first portion of the loop antenna within the first
wristband section. A reactance buffer is coupled to the second end
of the first conductor, the input buffer having a plurality of taps
linearly disposed along a longitudinal axis of the reactance
buffer. The distance between the outermost taps provides a
predetermined length corresponding to the loop antenna diameter
adjustment. A plurality of reactance elements is arranged
non-serially between the reactance buffer input and each of the
plurality of taps, wherein one or more of the reactance elements
are coupled between the input and a corresponding one of the taps
to provide a substantially constant reactance when measured between
the reactance buffer input and each of the plurality of taps. A
second wristband section includes a second conductor having a first
end coupled to the receiver ground and a second end. The second
conductor forms a second portion of the loop antenna within the
second wristband section. A coupling device couples to the second
end of the second conductor for coupling the conductor to one of
the plurality of taps. When the wristband length is adjusted by
selecting one of the plurality of taps, the resonant frequency of
the wristband loop antenna remains substantially unchanged.
It is an object of the present invention to provide a loop antenna
having an adjustable size which does not require tuning when the
size is changed.
It is a further object of the present invention to provide a loop
antenna which is adapted for use with a wristworn device.
It is a further object of the present invention to provide a
wristband loop antenna which can be pretuned.
It is a further object of the present invention to provide a
wristband loop antenna which when tuned is insensitive to changes
in the wristband length .
BRIEF DESCRIPTION OF THE DRAWINGS
The features of the invention which are believed to be novel are
set forth in particularity in the appended claims. The invention
itself, together with its further objects and advantages thereof,
may be best understood by reference to the following description
when taken in conjunction with the accompanying drawings, in which
the several figures of which like reference numerals identify
identical elements, in which:
FIG. 1 is a diagram of a prior art wristworn device utilizing a
wristband loop antenna.
FIG. 2A is an exploded view of one half of the adjustable strap
section of FIG. 1.
FIG. 2B is an electrical schematic diagram of FIG. 2A.
FIG. 3A is a diagram of a wristband loop antenna for the preferred
embodiment of the present invention.
FIG. 3B is a diagram showing a cross section of the wristband strap
showing the construction of the strap for the preferred embodiment
of the present invention.
FIG. 3C is a diagram of the construction of an inductive reactance
buffer for the preferred embodiment of the present invention.
FIG. 3D is a diagram showing a second cross section of the
wristband strap showing the construction of the strap for the
preferred embodiment of the present invention.
FIG. 4 is an diagram of a typical wristband loop antenna and an
equivalent electrical schematic diagram.
FIG. 5A is a diagram of the inductive reactance buffer for the
preferred embodiment of the present invention.
FIG. 5B is an electrical schematic diagram of the inductive
reactance buffer of FIG. 5A.
FIG. 6A is a diagram of a capacitive reactance buffer for an
alternate embodiment of the present invention.
FIG. 6B is an electrical schematic diagram of the capacitive
reactance buffer of FIG. 6A.
FIG. 7A is a diagram of the construction of the capacitive
reactance buffer of the alternate embodiment of the present
invention.
FIG. 7B and 7C are diagrams showing an alternate construction
embodiment of the capacitive reactance buffer.
Table I compares the performance of a loop antenna utilizing an
inductive reactance buffer to the performance of a prior art loop
antenna.
Table II illustrates the performance of a loop antenna utilizing a
capacitive reactance buffer.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With respect to the figures, FIGS. 3 to 6 illustrate the preferred
embodiment of the present invention, a buffered loop antenna
suitable for use with a wristworn electronic device. In order to
appreciate the advantages of the present invention, it is best to
describe in some detail the operation of at least one prior art
wristband loop antenna in order to provide an understanding of some
of the problems previously encountered. A typical prior art
wristband loop antenna arrangement 10 is shown in FIG. 1. The
receiver is located in housing 12 to which two non-stretchable
straps 14 and 16 are attached. Within each strap 14 and 16 is
located a conductor 18 and 20 respectively. This conductor may be
either a round or a flat conductive wire. Attached to one of the
wristband straps 14, a conventional buckle 22 is provided which
connects to one end of conductor 18. In the other wristband strap
16, a series of regularly spaced holes 24 (not shown) are provided
to allow for adjustment of the wristband length. An eyelet is often
inserted into each of the holes to provide electrical connection
with conductor 20 within strap 16. This is shown in greater detail
in FIG. 2A.
As shown in FIG. 2A, a wide flat sheet-metal conductor 100 is
located within strap 102. Eyelets 104 provide contact to conductor
100. The holes used to provide adjustment of the wristband are
marked T1 through T7 and are evenly spaced over a length of the
wristband, designated .DELTA.L. For a typical wristband, .DELTA.L
is approximately 44 millimeters in length for typical variations in
adult wrist size. A loop antenna constructed as shown in FIGS. 1
and 2A is an electrically small loop antenna, approximately
one-quarter wavelength in size at VHF frequencies. Such a loop
antenna is inductive at most frequencies of interest, and is
capacitively tuned. Consequently, the adjustable portion of the
wristband may be represented as a series of inductive elements, as
shown in FIG. 2B. The particular magnitude of the inductance of
each element is a function of the geometry, or size, of the
conductor, in this instance, the conductor geometry between each
tap T1 through T7. It will be appreciated, when the clasp is
connected to tap T1, the wristband size, which is also the relative
loop antenna size or diameter is a minimum. When the clasp is
connected to tap T7, the wristband size, or relative loop antenna
size or diameter is a maximum. Thus, it will be appreciated, when
the loop antenna is adjusted and tuned for length T1, the tuning
will be substantially changed at length T7, and for intermediate
lengths as well, resulting in reduced receiver sensitivity at
lengths other than where originally tuned.
FIGS. 3A, 3B, 3C and 3D show the general construction of a
wristband loop antenna for the preferred embodiment of the present
invention. As shown in FIG. 3A, the wristband loop antenna 200
includes two non-stretchable, but flexible straps, or wristband
sections 202 and 204. The first wristband section 202 includes a
first conductor 206 which forms a first portion of the loop
antenna, while the second wristband section 204 includes a second
conductor 208 and forms the second portion of the loop antenna. The
first wristband section 202 further includes a series of regularly
spaced apertures 210, such as holes or slots, linearly disposed
along the wristband to provide adjustability.
A standard two piece clasp, used widely in the watch industry is
utilized in the construction of the preferred embodiment of the
present invention. The clasp is suitably modified, such as with
plating, to minimize corrosion problems and to maintain low ohmic
electrical contact when the clasp is secured. Platings, such as
selective gold plating of the contact surfaces is preferred,
although other plating techniques may be employed equally as well.
Adjustable clasp 212 is slidably positioned along wristband section
202, and provides electrical contact to first conductor 206.
Attached to the end of the second wristband section 204 is a fixed
clasp 214, which couples to one end of second conductor 208, and
together with adjustable clasp 212 provides the means to both
electrically complete the loop antenna, and to mechanically secure
the wristband 200 to the wrist. First wristband section 202 and
second wristband section 204 are affixed to the wristworn device by
an attachment means, such as rigid mounting brackets 216, which are
secured to the device housing by fasteners, such as screws (not
shown). Mounting brackets 216 may be formed from sheet metal, such
as stainless steel, or other suitable material which is generally
unaffected by contact with the skin. Stainless steel is
advantageous in not requiring any plating for providing corrosion
resistance. It will be appreciated, the rigid mounting of the
wristband sections is exemplary and that other attachment means,
such as the use of watch style spring loaded pins, may be used as
well.
In the preferred embodiment of the present invention, conductor 208
is a flat sheet-metal conductor formed from half hard beryllium
copper material which is 3-4 mils thick. Other materials such as
copper, nickel silver, and other conductive materials may be used
as well. Conductor 208 is generally continuous through the length
of wristband section 204, coupling on one end to the fixed clasp
214 and to a receiver input, such as the receiver ground input, at
the device housing. Conductor 208 may be formed in a manner shown
in FIG. 3C to provide positive retention of the conductor within
the body of wristband section 204.
FIG. 3B and 3C shows the construction details for the first
wristband section 202. In the preferred embodiment of the present
invention, wristband section 202 is constructed by laminating
conductor 206 and reactance buffer 218, which will be described in
detail shortly, between top 220 and bottom 222 members which are
non-stretchable, flexible materials formed by any number of
suitable methods, such as by injection molding or die cutting, as
shown in FIG. 3B. Any number of materials may be used for the top
220 and bottom 222 members, such as a urethane rubber, leather and
the like. The bottom member 222, or the top member 220, may include
a recessed area, such as recess 224, in which conductor 206,
reactance buffer 218, and mounting bracket 216 are positioned. Such
a recessed area can be formed in the material when the strap is
molded. As shown in FIG. 3C, conductor 206 has an bent conductor
portion 226 which is used to retain the conductor in the recess and
prevents the conductor from pulling out or moving in the finished
wristband section When it is impractical to provide a recess,
adhesives may be utilized to provide the retention of the
conductor. Depending on the material of the two members, the two
members may be joined by such processes as chemical bonding,
including solvents and adhesives; mechanical bonding, including
thermal, and ultrasonic bonding; and stitching or adhesive bonding,
as in the case of a leather wristband. Insert molding of complete
wristband sections may also be used, thereby eliminating many of
the secondary wristband assembly operations described. Conductors
206 and 208 are formed from flat sheet metal using such methods as
stamping, chemical etching, or other suitable process. Mounting
bracket 216 is retained within a cavity formed within a portion of
the wristband section, as shown in the cross-sectional diagram of
FIG. 3D. Mounting bracket 216 is retained within the cavity by any
of a number of well known methods, such as with the use of serrated
edges formed as part of the bracket, as illustrated.
FIG. 4 shows a diagram of a wristband antenna and an equivalent
electrical schematic diagram which is useful in describing the
operation of both the prior art wristband loop antenna, and the
buffered loop antenna of the present invention. As previously
described, the wristband loop antenna formed by bands A and B are
inductive at the operating frequency, indicated schematically as
L.sub.(b-x), the subscript denoting the plurality of inductances as
the length of the loop is adjusted (x indicating position T.sub.1
to T.sub.7 and b indicating the reference end of the second band as
shown in FIG. 4). The resistance associated with the conductors is
shown schematically as R.sub.s. The wristband loop antenna couples
to a receiver input and ground as shown, and is capacitively tuned,
the capacitor shown schematically as C.sub.o. In the preferred
embodiment of the present invention, capacitor C.sub.o couples
between the receiver input and ground. The voltage delivered from
the loop antenna operating in an electromagnetic field is shown
schematically as the voltage source labeled E.
The operating frequency of the antenna may be determined by the
following well known equation.
From the previous description of FIGS. 2A and 2B, it was noted the
inductance at tap T1, does not equal the inductance at the other
taps. Thus
where L.sub.(b- 1), etc. represents the magnitude of the total
inductance measure at each tap position. The total inductance of
the loop antenna is the sum of the inductance of band A and band B,
corrected for the differential inductance associated with varying
the length of the loop in the adjustable zone.
It then follows, if C.sub.o is kept constant, such as when the
capacitor is pretuned at one of the wristband lengths, then
which demonstrates, as previously stated, the prior art wristband
loop antenna requires retuning to eliminate variations in adjusting
the wristband to different wrist sizes. This problem is
substantially minimized with the reactance buffer described in FIG.
3B, the operation of which will be described in detail with FIGS.
5A and 5B. In practice, the reactance buffer of the present
invention provides substantially a constant reactance for each tap
position along the wristband, such that
which results in
The reactance buffer for the preferred embodiment of the present
invention, by providing a substantially constant reactance at each
tap position, allows the wristband loop antenna to be tuned only
once at any of the selectable wristband lengths, and thereafter the
wristband loop antenna remains tuned, even when the diameter of the
antenna loop is changed.
FIG. 5A shows a diagram of the physical layout of the reactance
buffer 218 for the preferred embodiment of the present invention.
An approximate schematic diagram of reactance buffer 218 is shown
in FIG. 5B. It will be appreciated, that the schematic diagram of
FIG. 5B is only a first order approximation for the reactance
buffer, in that each conductor in the circuit has an associated
inductance value. The schematic diagram of FIG. 5B represents
inductance values associated with horizontal conductors. While the
vertical conductors also have inductance values associated with
them, they are shown schematically as conductors, or conductive
elements. It will be appreciated, this first order approximation is
sufficient to one of ordinary skill in the art to understand the
operation of the reactance buffer 218 to be described.
Reactance buffer 218 is an integrated structure, as shown in FIG.
5A in that the buffer input, the taps, and the reactance elements
are formed from a flat sheet metal strip. The taps are linearly
disposed along the integrated structure providing buffer outputs to
select the wristband size. The outermost taps, T1 and T7, are
spaced a predetermined length, corresponding to the amount of
wristband size adjustment required.
Referring to FIG. 5B, first conductor 206 is shown schematically as
inductor L1. Reactance buffer 218 input is shown generally as
conductor 300. Reactance buffer 218 includes a plurality of taps
T1-T7 which are used to adjust the length of the wristband, or
conversely, the diameter of the wristband loop antenna. It will be
appreciated, the number of taps provided for the adjustment range
is for example only, and other numbers may be provided when
necessary. Reactance buffer 218 comprises a plurality of reactance
elements, shown schematically as inductive elements, or inductors,
L2-L10. The arrangement, i.e. series/parallel combinations of these
reactance elements, results in a substantially constant reactance
when measured between the buffer input 300 and each of the taps
T1-T7. As shown, each inductive element is in actuality a
conductor, the value of the inductance being a function of the
geometry of the inductor. Thus, L2 which corresponds to conductor
304, has a substantially equivalent inductance value to L3 which
corresponds to conductor 306. Inductance values at other taps are
combinations of inductances corresponding to a number of series and
parallel inductors, as shown.
Table I illustrates the relative performance of the inductive
reactance buffer compared to the prior art loop antenna design. All
measurements are referenced to tap T1, and includes a conductor
length equivalent to that found in the first antenna portion. The
relative length is the additional length of the wristband, as the
wristband is adjusted from T1 to T7. The inductance change is the
change in inductance value associated with each tap relative to the
inductance reference measure at T1. The total inductance and change
in inductance for the prior art antenna are tabulated in the last
two columns of Table I. As Table I shows, the change in inductance
for the prior art antenna was measure at 59.1 nanohenries, compared
to a maximum change of 4.3 nanohenries. It will be appreciated that
further optimization of the conductor geometries in the reactance
buffer can be made to reduce this difference.
As shown in FIGS. 3B and 5A, reactance buffer 218 may be
advantageously and economically formed from a single flat sheet
metal conductor which has been formed, such as by die stamping or
chemical etching. It will be appreciated, the conductor pattern
shown is, for example, only, and any number of conductor patterns
may be generated which achieve the same result, a substantially
constant reactance measured between the buffer input and each
output tap. The conductive pattern may be formed from sheet metal,
such as copper, beryllium copper and nickel silver. The material is
selected to provide the required flexibility, and to withstand the
repeated flexing associated with wearing the wristband and
repeatedly putting on and removing the wristband from the wrist.
The conductor may be plated to enhance the solderability, and
durability of the conductor, with a plating such as a copper,
nickel, tin plating.
Other materials for forming the reactance buffer may also be
employed, other than described above. One such material may be a
copper foil laminated KAPTON material, wherein the reactance buffer
pattern is formed using convention printed circuit etching
techniques. Coupling of the pattern to the tap areas would be the
same, or similar to the stamped metal reactance buffer, such as
with rivets.
Alternate construction methods for the reactance buffer is shown in
FIGS. 6A/6B and 7A-7C. The reactance buffers of FIGS. 6A/6B and
7A-7C utilize a plurality of fixed value capacitors to achieve a
substantially constant reactance when the length of the wristband
is adjusted. As shown in FIG. 6A, a portion of conductor 206 is
tapped using conductors 400-412, somewhat in the method of the
prior art. However, unlike the prior conductor 206 is coupled to
each output tap T1-T7 through a fixed capacitor C1-C7. FIG. 6B
shown an approximate schematic diagram of FIG. 6A. In the instance
where both inductive and capacitive elements are utilized in the
reactance buffer, the reactance elements may be considered to
include a plurality of paired inductive and capacitive elements,
such as L11 and C1. Each inductive and capacitive element has an
input and an output, the input of the capacitive element being
coupled to the output of the inductive element, and the output of
the capacitive element being coupled to a tap. The inductive
elements are then coupled in series, resulting in the structure
shown in FIG. 6B. The values for C1-C7 are selected to provide a
substantially constant reactance between the input and each output
tap, the magnitude of this capacitance being computed as
follows:
where f is the frequency of operation, L.sub.cum is the cumulative
inductance associated with each tap, and C.sub.tap is the
particular tap capacitance. Thus, L.sub.cum would equal L11 L12,
and C.sub.tap would be C2 for tap T2. Thus, C1, when used, would
have the smallest capacitance value for resonating with inductor
L11, whereas C7 would have the largest capacitance value for
resonating with the series combination of L11-L17. While capacitor
C1 is shown, it will be appreciated C1 can be omitted with the
buffer retaining the same electrical characteristics previously
described, in which case C2 would have the smallest inductance
value resonating with L.sub.11 and L.sub.12.
One construction method for a reactance buffer utilizing capacitive
and inductive elements is shown in FIG. 7A. A flexible circuit 508,
such as a KAPTON film with laminated copper foil is first etched to
provide a pattern similar to shown in FIG. 6A. Capacitors C4-C7,
such as leadless, surface mountable chip capacitors, having
appropriate values are then soldered, such as using reflow
soldering, to attach the capacitors to the conductors. A molded, or
die cut, elastomer or leather band is then assembled enclosing the
flexible circuit using one of more of the procedures previously
described for the inductive reactance buffer of FIGS. 3A and
3B.
Table II illustrates the relative performance of the
capacitive/inductive reactance buffer. All measurements are
referenced to tap T1, and includes a conductor length equivalent to
that found in the first antenna portion. The relative length is the
additional length of the wristband, as the wristband is adjusted
from T1 to T7. The total inductance is listed for three tap
positions. Cadded is the computed capacitance required to resonate
the total inductance at each tap to a predetermined operating
frequency, which in the case of this example is 157.7 MHz. As table
II shows, proper selection of fixed value capacitors at each tap
can substantially eliminate any changes in antenna tuning, as the
length of the wristband is changed.
An alternate construction for the capacitive reactance buffer is
shown in FIGS. 7B and 7C. In this instance, the capacitors are
formed during the construction of the wristband section 202. As
shown in FIG. 7B, one plate of capacitors C1-C7 is coupled with a
contact 500. The size of the plate 500 is a function of the
capacitance required at each tap, the thickness of dielectric layer
502, and the dielectric constant of dielectric layer 502.
Computation of the size of the capacitor plate is well known to one
of ordinary skill in the art. The second plate of each of the
capacitors C1-C7 is provided by conductor 206. In practice,
capacitor plate/contacts 500 are placed in a molded wristband half
504. Each capacitor plate/contact has a different geometry
corresponding to the required capacitance at each tap. Dielectric
layer 502 is positioned over the contacts, followed by the
positioning of conductor 206. Dielectric layer 502 may be molded
from a suitable dielectric, having a recess in which to position
conductor 206. Finally, the top wristband half 510 is positioned on
the stack, and the combination laminated by one or more appropriate
techniques previously described for the inductive reactance buffer
construction.
As shown in FIG. 7C, wristband section 204 may be constructed to
provide connection to the capacitor/inductor buffer. In this
instance, conductor 208 may be formed, such as by stamping or
coining techniques, to form contacts 506 to be plugged into
capacitor plate/contacts 500. Two contacts are shown in this
alternate embodiment of the present invention. The two contact
arrangement provides additional strength to the clasp when the
clasp is secured as well as a more reliable electrical contact.
Other methods of forming the contact on conductor 502 may also be
employed, such as by attaching separate fixed contacts.
As in the case of the inductive buffer of FIG. 5A, the capacitive
buffers of FIGS. 7A-7C may be described as a flat integrated
structure which includes the buffer input, taps and reactance
elements.
While the description of the buffered loop antenna has been
directed primarily for use in a wristband, it will be appreciated,
the reactance buffer of the present invention can be used in other
loop antenna applications as well. Examples of such applications,
include any variable size loop antenna, either electrically small
or electrically large and having any cross sectional configuration,
such as circular, square, rectangular or other. Other applications
include such special purpose variable size loop antennas, such as
could be located in belts, rigid bracelets, ankle straps, and the
like.
While specific embodiments of the present invention have been shown
and described, further modifications and improvements will occur to
those skilled in the art. All modifications which retain the basic
underlying principles disclosed and claimed herein are within the
scope and spirit of the present invention.
TABLE I
__________________________________________________________________________
Buffered Band Antenna Band Antenna Band's Relative Relative
Relative Position Length Inductance Inductance Inductance
Inductance B (x) .DELTA.I = B (x) - B (1) L(nH) .vertline..DELTA.L
= L (B - 1) - L (B - x).vertline. L(nH) .vertline..DELTA.L = L (B -
1) - L (B
__________________________________________________________________________
- x).vertline. B-1 0 mm 193.7 0.0 nH 142.4 nH 0 nH B-2 9.04 mm
192.1 1.6 nH -- -- B-3 16.05 mm 192.6 1.1 nH -- -- B-4 23.06 mm
195.9 2.2 nH 168.1 nH 25.7 nH B-5 30.07 mm 192.4 1.3 nH -- -- B-6
37.08 mm 189.4 4.3 nH -- -- B-7 44.09 mm 196.4 2.7 nH 201.5 nH 59.1
nH
__________________________________________________________________________
nH = nanohenries
TABLE II ______________________________________ Relative Band's
Length Inductance Position .DELTA.L = B (x) - B (1) L (B - x)
C.sub.added F.sub.ant ______________________________________ B-1 0
mm 142.4 nH 0 pF 157.75 MHz B-2 9.04 mm -- -- -- B-3 16.65 mm -- --
-- B-4 23.05 mm 168.1 nH 39.8 pF 157.75 MHz B-5 30.07 mm -- -- --
B-6 37.08 mm -- -- -- B-7 44.09 mm 201.5 nH 17.2 pF 157.75 MHz
______________________________________ nH = nanohenries
* * * * *